1. Field of the Invention
The present invention relates to lighting devices using rod integrators which combine multiple light beams, and in particular to projection type high-brightness display apparatuses which use image display elements.
2. Description of the Background Art
As a method to display a large-screen view, projection type display apparatuses are known which illuminate image display elements with light in accordance with video signals to form optical images, and which project and enlarge the optical images through projection lenses. The image display elements are typified by liquid crystal panels and digital mirror devices (DMDs). In such projection type display apparatuses, improvement in brightness of the projected images is strongly demanded.
To improve the brightness of projected images from a projection type display apparatus, power consumption of lamp used as the light source can be increased. However, if the power consumption is increased while a sufficiently long lifespan of the lamp is ensured, light-emitting elements thereinside have to be elongated and thickened. As a result, light use efficiency of the optical system will deteriorate. In the case of using LEDs instead of lamps, even if the power consumption of the projector is to be increased to a predetermined level of power consumption, it is not possible to ensure the long lifespan of the light source (LEDs), since the upper limit of a current value the LEDs can withstand is not high. In addition, in this case, it is difficult to minimize the heat generated from the LEDs within a range the LEDs can withstand. Thus, to solve the above problems, techniques are being developed which use a plurality of light sources having relatively small power consumptions, such as LEDs, thereby to efficiently improve the brightness of the projection type display apparatuses.
As shown in FIG. 6, in Japanese National Phase PCT Laid-Open Publication No. 2001-512584 (Patent Literature 1), light beams outputted from two light sources 401 and 402 are combined using a rod integrator, and thereby high brightness is realized. Prisms 405 and 406 are arranged so that they are in contact with an incident surface 404 of a composite rod 403. Between the prism 405 and the light source 401, a converging lens 407 is arranged so that parallel light beam emitted from the light source 401 are focused on the incident surface 404 of the composite rod 403. Similarly, between the prism 406 and the light source 402, a converging lens 408 is arranged so that parallel light beam emitted from the light source 402 are focused on the incident surface 404 of the composite rod 403. The light beam emitted from the light source 401 enters the prism 405 through the converging lens 407, is reflected by an inclined surface 409, and form a spot on the incident surface 404 of the composite rod 403. Similarly, the light beam emitted from the light source 402 enters the prism 406 through the converging lens 408, is reflected by an inclined surface 410, and forms a spot on the incident surface 404 of the composite rod 403. The light beams emitted from the light sources 401 and 402 are repeatedly reflected inside the composite rod 403, and thereby combined together. Accordingly, light having uniform luminance distribution is outputted from an exit surface 411 of the composite rod 403.
As shown in FIG. 7, in Japanese Laid-Open Patent Publication No. 2008-191653 (Patent Literature 2), light beams emitted from two light sources 501 and 502 enter a triangular prism 503 perpendicularly to a first surface 504 and a second surface 505. The light beam emitted from the light source 501 passes through the first surface 504, is totally reflected by the second surface 505, and enters the composite rod 506. On the other hand, the light beam emitted from the light source 502 passes through the second surface 505, is totally reflected by the first surface 504, and enters the composite rod 506. The apical angle of the triangular prism 503 is preferably 60 degrees so that the totally reflected light beams travel in parallel to each other.